As glaciers scour the landscape, they pick up massive boulders and move
them far from their point of origin. When the glacier that formed Glacial
Lake Missoula's dam burst, uncountable boulders embedded in its ice were
carried along with the floodwaters. As these ice blocks melted, the boulders
dropped out of the flow. Today, boulders with origins in the mountains
of Montana and British Columbia can be found along the flood paths as
far away as Oregon's Willamette Valley.